add ComboBox node, add blend modes button in stroke panel, move brush shader code to ShaderManager and avoid the same shader being recompiled many times

engine/shader.cpp

@@ -171,6 +171,13 @@ void ShaderManager::u_int(kShaderUniform id, int i)
 {
     m_current->u_int(id, i);
 }
+
+Shader* ui::ShaderManager::get(kShader id)
+{
+    auto it = m_shaders.find(id);
+    return (it == m_shaders.end()) ? nullptr : &it->second;
+}
+
 void ui::ShaderManager::u_float(kShaderUniform id, float f)
 {
     m_current->u_float(id, f);